This invention relates to thermistor chips with reduced fluctuations in the resistance values. This invention also relates to methods of making such thermistor chips.
As shown in FIGS. 14 and 15, conventional thermistor chips 1 are usually produced by forming electrodes 3 at both end parts of a thermistor chip block 2 having a negative temperature characteristic (NTC) made of a fired ceramic material having an oxide of a transition metal such as Mn, Co and Ni as its principal component. The electrodes 3 each comprise a first metal layer 3a formed by applying a paste of Ag or Ag/Pd on the end parts of the thermistor chip element 2 and then firing on it and a second metal layer 3b formed by applying a solder material on the surface of the first metal layer 3a.
Recently, thermistor chips of this kind are required to be miniaturized. From the point of view of resistance values, demands for thermistor chips with low resistance values are growing. Many problems arise, however, if one attempts to reduce the size of a thermistor chip as well as its resistance value. For example, small thermistor chip elements are difficult to handle, they are thin and they crack easily. As the separation between the electrodes 3 at both ends (indicated by letter "a" in FIG. 15) is reduced, a bridge-like structure of solder is likely to form.
For improving the efficiency of production, thermistor chip elements of the same size are sometimes used to produce thermistor chips with different resistance values by varying the size of the electrodes. In such a situation, the width of the electrodes 3 (indicated by letter "d" in FIG. 15) often becomes non-uniform, and it becomes necessary to provide land connectors with different shapes corresponding to different values of d. Depending on the shape of the connecting land, furthermore, the thermistor chip may even be caused to stand up at the time of soldering (or the formation of so-called "tombstones").
Moreover, there are generally large fluctuations in the normal-temperature resistance values (hereinafter simply referred to as the resistance values) of thermistors determined by the resistance of the thermistor chip element itself and the positions of the terminal electrodes 3. The so-called "3cv" value (an index of fluctuations defined as 100.times.3 .sigma./(average value) where .sigma. indicates the standard deviation of fluctuation within a lot) for the resistance values of thermistor chips is conventionally as large as 5-20% and it was too costly to obtain products with a smaller deviation of less than 1%.